Photonic, photovoltaic, optical and micro-mechanical devices are typically packaged such that the active elements (i.e., the emitters, receivers, micro-mirrors, etc.) are disposed within a sealed chamber to protect them from handling and other environmental hazards. In many cases, it is preferred that the chamber be hermetically sealed to prevent the influx, egress or exchange of gasses between the chamber and the environment. Of course, a window must be provided to allow light or other electromagnetic energy of the desired wavelength to enter and/or leave the package. In some cases, the window will be visibly transparent, e.g. if visible light is involved, but in other cases the window may be visibly opaque while still being “optically” transparent to electromagnetic energy of the desired wavelengths. In many cases, the window is given certain optical properties to enhance the performance of the device. For example, a glass window may be ground and polished to achieve certain curve or flatness specifications in order to disperse in a particular pattern and/or avoid distorting the light passing therethrough. In other cases, anti-reflective or anti-refractive coatings may be applied to the window to improve light transmission therethrough.
Hermetically sealed micro-device packages with windows have heretofore typically been produced using cover assemblies with metal frames and glass window panes. To achieve the required hermetic seal, the glass window pane (or other transparent window material) has heretofore been fused to its metallic frame by one of several methods. A first of these methods is heating it in a furnace at a temperature exceeding the window's glass transition temperature, TG and/or the window's softening temperature TS (typically at or above 900° C.). However, because the fusing temperature is above TG or TS, the original surface finish of the glass pane is typically ruined, making it necessary to finish or re-finish (e.g., grinding and polishing) both surfaces of the window pane after fusing in order to obtain the necessary optical characteristics. This polishing of the windowpanes requires additional process steps during manufacture of the cover assemblies, which steps tend to be relatively time and labor intensive, thus adding significantly to the cost of the cover assembly, and hence to the cost of the overall package. In addition, the need to polish both sides of the glass after fusing requires the glass to project both above and below the attached frame. This restricts the design options for the cover assembly with respect to glass thickness, dimensions, etc., which can also result in increased material costs.
A second method to hermetically attach a transparent window to a frame is to solder the two items together using a separate preform made of a metal or metal-alloy solder material. The solder preform is placed between a pre-metallized window and a metal or metallized frame, and the soldering is performed in a furnace. During soldering, no significant pressure is applied, i.e., the parts are held together with only enough force to keep them in place. For this type of soldering, the most common solder preform material is eutectic gold-tin.
Eutectic gold-tin solder melts and solidifies at 280° C. Its CTE at 20° is 16 ppm/° C. These two characteristics cause three drawbacks to the reliability of the assembled window. First, the CTE of Mil-Spec kovar from 280° C. to ambient is approximately 5.15+/−0.2 ppm/0 C, while most window glasses intended for sealing to kovar have higher average CTEs over the same temperature range. During cooling from the set point of 280° down to ambient, the glass is shrinking at a greater rate than the kovar frame it's attached to. The cooled glass will be in tension, which is why it is prone to cracking. To avoid cracking, the glass should have an identical or slightly lower average CTE than the kovar so as to be stress neutral or in slight compression after cooling. Using solders with lower liquidus/solidus temperatures puts the kovar at a higher average CTE, more closely matching the average CTE of the glass. However, this worsens the second drawback of metal-allow solder seals.
The second drawback to soldering the glass to the kovar frame is that the window assembly will delaminate at temperatures above the liquidus temperature of the employed solder. Using lower liquidus/solidus temperature solders, while reducing the CTE mismatch between the kovar and glass, further limits the applications for the window assembly. Most lead-free solders have higher liquidus/solidus temperatures than the 183° C. of eutectic Sn/Pb. Surface-Mount Technology (SMT) reflow ovens are profiled to heat Printed-Wiring Board (PWB) assemblies 15-20 degrees above the solder's liquidus/solidus temperature. So the SMT reflow-soldering attachment to a PWB of a MOEMS device whose window was manufactured using lower melting-point solder preforms might have the unfortunate effect of reflowing the window assembly's solder, causing window delamination.
The third drawback is that the solder, which is the intermediate layer between the glass and the kovar frame, has a CTE up to three times greater than the two materials it's joining. An intermediate joining material would ideally have a compensating CTE in-between the two materials it's bonding.
A third method to hermetically attach a glass window to a frame is to solder the two items together using a solder-glass material. Solder-glasses are special glasses with a particularly low softening point. They are used to join glass to other glasses, ceramics, or metals without thermally damaging the materials to be joined. Soldering is carried out in the viscosity range h where his the range from 104 to 106 dPa s (poise) for the solder-glass; this corresponds generally to a temperature range T (for the glass solder or solder-glass) within the range from 350° C. to 700° C.
Once a cover assembly with a hermetically sealed window is prepared, it is typically seam welded to the device base (i.e., substrate) in order to produce the finished hermetically sealed package. Seam welding uses a precisely applied AC current to produce localized temperatures of about 1,100° C. at the frame/base junction, thereby welding the metallic cover assembly to the package base and forming a hermetic seal. To prevent distortion of the glass windowpane or package, the metal frame of the cover assembly should be fabricated from metal or metal alloy having a CTE (i.e., coefficient of thermal expansion) that is similar to that of the transparent window material and to the CTE of the package base.
While the methods described above have heretofore produced useable window assemblies for hermetically sealed micro-device packages, the relatively high cost of these window assemblies is a significant obstacle to their widespread application. A need therefore exists, for package and component designs and assembly methods which reduce the labor costs associated with producing each package.
A need still further exists for package and component designs and assembly methods that will minimize the manufacturing cycle time required to produce a completed package.
A need still further exists for package and component designs and assembly methods that reduce the number of process steps required for the production of each package. It will be appreciated that reducing the number of process steps will reduce the overhead/floor space required in the production facility, the amount of capital equipment necessary for manufacturing, and handling costs associated with transferring the work pieces between various steps in the process. A reduction in the cost of labor may also result. Such reductions would, of course, further reduce the cost of producing these hermetic packages.
A need still further exists for package and component designs and assembly methods that will reduce the overall materials costs associated with each package, either by reducing the initial material cost, by reducing the amount of wastage or loss during production, or both.
Many types of multi-pane insulated window assemblies are known. A conventional multi-pane insulated window assembly consists, at a minimum, of two windowpanes joined by a frame that maintains a space between them. The space is filled with air or another thermally insulating material, typically a gas. Multi-pane insulated window assemblies typically have better thermal insulation properties than single-pane windows, however, further improvement in insulating performance is often desired.
A vacuum-glazing unit (VGU) is a window assembly similar to a multi-pane insulated window assembly, except a vacuum or partial vacuum is maintained in the space between the windowpanes. The purpose of this type of construction is to produce an insulated window unit with a higher level of thermal insulation that can be obtained from air- or gas-filled insulated window assemblies. To date, however, many problems have been experienced in producing durable and reliable VGUs. For example, it has proven difficult to achieve seals between the windowpanes and the frame having the hermeticity necessary to maintain a vacuum (or partial vacuum) for an extended period. Further, it has proven difficult to produce VGUs for exterior wall installations (i.e., for use in the outside-facing (exterior) walls and doors of buildings) that can withstand large and/or rapid thermal cycling (e.g., caused by changes in outside temperatures and/or use of high-performance HVAC systems) without eventually leaking or cracking. A need therefore exists, for improved VGUs and methods of producing durable and reliable VGUs suitable for use in exterior walls and doors, as well as for other applications.
A Jun. 10, 2005 Department of Energy (DOE) solicitation states that the key technical challenges associated with highly insulating fenestration products include, but are not limited to: larger size (˜25 sq. ft. and larger), improved durability, excessive weight, seal durability, and high cost. Without an aggressive program to change the energy-related role of windows in buildings, it will thus be virtually impossible to meet Zero Energy Buildings goals. The DOE's Window Technology Industry Roadmap (Roadmap), published by the Office of Building Technology, State and Community Programs (BTS), after listing several areas of window technology in need of improvements, states such improvements have not been realized due to factors including: High-first-cost of improved products; the cost and questionable durability of existing highly-insulating window technologies; the lack of industry collaboration to improve insulation technology and manufacturing methods; and the presumed high-risk-low-return ratio of investments in improved technologies.
In fact, the window industry has not improved the basic technology or reliability of insulating windows for decades. Manufacturers use an adhesive to bond pairs of windowpanes to an intermediate spacer to achieve an airtight cavity between the windowpanes. No epoxy, glue or other adhesive in use today is airtight. All permit some amount of gas exchange to occur. According to data published in 2002 by The Sealed Insulated Glass Manufacturers Association (SIGMA), warranty claims for installed insulated glass (IG) window units due to seal failures is 4% ten years after installation, and almost 10% fifteen years after installation. Most window units do not identify the manufacturer. Many homeowners consciously or inadvertently choose to live with the failed window seals and water condensation between the IG windowpanes that reduce energy efficiency. The majority of IG unit (IGU) seal failures are not considered in the SIGMA data. The actual number of IGU seal failures 15 years after installation is unknown and believed to be very high. All of these conditions are bleeding us of energy.
Some academic institutions, companies and government labs have tried achieving higher insulating values (higher R-value; lower U-value) while attempting to solve the issue of leaking seals. Their solutions all have four things in common: The units contain a vacuum between windows #1 and #2 to provide higher insulation than a fill gas; mechanical spacers are used to maintain the separation of the window lites (i.e., panes) #1 and #2 (if the lites come in physical contact with each other, this creates an undesirable thermal path that substantially reduces the IG unit's insulating value); the lites are hermetically sealed at their perimeters (most commonly, using reflowed solder glass to seal two closely separated lites, and less commonly, using a laser to melt the two lites together); and all currently produced or described vacuum glazing units employ a tube (i.e., “pinch-tube”) to evacuate the IG unit, after which the tube is sealed shut.
These experimental solutions are not commercially available in the U.S. because they have failed or have not proven to be reliable. Problems include: the spacers are opaque or not aesthetically appealing so they fail to meet industry needs; laser attempts at sealing have resulted in broken lites due to thermal shocking of the glass; high thermal conductivity between the perimeter surfaces of the inside of the glass lites where they are sealed together; stress eventually causes either the seal or the lites to break because the sealing method is not compliant (flexible); elevated soldering temperatures eliminate the ability to use some soft-coat low-e coatings; and/or when a vacuum tube is added, it increases the unit's complexity and decreases its reliability.
A need therefor exists, for vacuum glazing units (VGUs) and insulated glass units (IGUs) having improved designs which address some of the aforesaid problems with the current technology.